Highlighting sequence is slow and confusing

[haydnKing]

slow because it's overcomplicated (lots of DOM faffing), confusing because it appears as though you're only highlighting the forward strand, where you're actually selecting both

[mrginglymus]

Drag-to-select highlighting isn't really what people want to do - they want to select sequence data defined by either bp location or features - I wouldn't spend too much time improving that performance, but instead let users copy sequence data based on those two options.